The present invention relates generally to combustion systems, and more particularly to combustion systems having an arrangement for suppressing combustion chamber vibrations.
There are many instances, for example in industrial furnace recuperators, where combustion chamber vibrations of the organ-pipe variety develop when the system is in operation. Such vibrations can lead to damage to the system. Details of the development and the possible damage resulting from such vibrations can be found in A. A. Putnam and W. R. Dennis, "Survey of Organ-Pipe Oscillations in Combustion Systems," Journal of the Acoustic Society of America, 28 (1956). These are self-excited acoustic vibrations in the gas columns in the combustion chamber, in the grate chamber, and in the conduits which supply the fuel liquid and the combustion air to the burner. The self-excitation of such oscillations or vibrations is largely determined by the characteristics of the flame in the burner, the size and form of the combustion chamber and the acoustic impedances of the outlets of the fuel fluid and combustion air conduits into the combustion chamber. Details of the determination and calculation of the acoustical impedance may be found in E. Meyer and E. G. Neumann, "Physikalische und Technische Akustik", Friedrich Wieweg & Sohn, Braunschweig, Germany, 1967.
The development of such oscillations is undesirable, not only because of the noise level involved, but because they can lead to actual damage to the system. It has been found that of the various factors which are involved in the generation of such oscillations, the acoustical impedances are most readily susceptible to variation, and thus can be used in an attempt to control the development of the oscillations.
In fact, in a research paper issued by IRSID, namely J. M. Pariel, und L. de Saint Martin, "Contribution a l'Etude' des Instabilites de Combustions dans les Foyers Industriels," Revue Generale de Thermique, Volume VI, No. 69, Sept. 19, 1967, recommendations are made for selecting the length of the conduits between the burner inlet and acoustically reflecting points of the conduit system in such installations, in an attempt to overcome the development of oscillations. Acoustically reflecting points are identified as the change in cross-section between the gas collecting conduit and the feeder line extending from the same to the burner chamber, and the blower for supplying combustion air. The influence of such devices as the damper or throttle in the fuel fluid conduit, which extend into the cross-section of the conduit, is not taken into account in these recommendations. It is intended that when the recommended distances between the burner inlet and the main reflective points are maintained, the acoustic impedances of the conduits are to assume values at the burner inlet which prevent the development of self-excited oscillations.
However, experience and examinations have shown that in actual fact even installations constructed in accordance with these requirements are still subject to self-excited combustion chamber oscillations which, as examination has shown, are the result primarily of the acoustic behavior of components which extend into the cross-section of the fuel fluid feeder conduit intermediate the burner and the variation in the cross-section which occurs where the feeder conduit branches off from the collecting conduit. These devices, such as the throttle which is used to vary the flow of fuel fluid, and a measuring device for measuring the flow of fuel fluid, are absolutely necessary for a proper operation of the system and it is therefore not possible to dispense with them. A further reason which has been found for the development of self-excited oscillations despite the recommendations made in the industry is that these recommendations assume an excessively high impedance range as having a vibration damping characteristic, which is not the case.